Not so James, who by his lack of determination and his hesitancy appeared to be resolved to destroy any hope of success which his army cherished. One moment he decided on a general retreat, and for that purpose ordered the camp to be raised; but the next minute he altered his plan, and, sending off the baggage and six of his twelve field-pieces to Dublin, he apparently made up his mind to risk a battle. The removal of the baggage was no doubt a good preparation for an orderly retreat, but it was also a plain intimation to the army that a retreat was in contemplation; otherwise the reduction of the artillery must be considered a fatal diminution of strength. James indeed seems to have thought of nothing so much as means whereby to keep open a passage in the rear; and all his anxiety appears to have been lest William's forces should by a flank movement cut off his retreat to the south, where he had already privately made preparations for his flight to France. It is evident, also, that he resolved to place himself in such a position during the battle that he would be one of the first to see on which side fortune turned, so that in case of defeat he might with ease make good his escape. Still, with such apprehensions, it is strange how difficult it proved to persuade him to take any precautions for the defence of the fords up the river; for late on the eve of the battle he could only be induced with much difficulty to send Sir Neil O'Neill with his regiment of dragoons to defend the Pass of Rossnaree, about four miles from the Jacobite camp towards Slane.

William also called a council of war, at nine o'clock in the evening, not to take the advice of his officers, but to convey to them the fact that he had resolved to force the passage of the river next morning. Rendered impatient by news he had received of political intrigues in England, and apprehensive, from reports that had reached him from the Jacobite camp, that James would retreat, he would not listen to Schomberg's urgent advice against an enterprise that appeared to be very hazardous. Determined that his plans should not be known, and suspicious of the fidelity of some of his officers, he merely announced that he would send to the tent of each officer that night his particular orders. Schomberg, who appears to have been ignorant of the motives of William, is said, when the order of battle was delivered to him, to have remarked that he was more used to giving such orders than to receiving them. The old general is also said to have been much annoyed at the overruling of his advice to detach a portion of the army to secure the bridge of Slane, so as to turn the flank of the Jacobites and cut them off from the Pass of Duleek. At midnight William made a final inspection of his forces by torchlight, and issued his final orders, which included directions that "every soldier was to put a green bough in his hat. The baggage and great coats were to be left under a guard. The word was 'Westminster'."

The morning of Tuesday, the 1st of July, 1690, dawned bright and unclouded on the hostile camps. The first movement in the Williamite army was the march, at sunrise, of a division of 10,000 picked men. William's orders were that the river should be crossed in three places. The right wing, commanded by Count Meinhardt Schomberg (one of the Duke's sons), assisted by Lieutenant-General Douglas and Lord Portland, was to pass at some fords near the bridge of Slane to turn the left flank of the Jacobite army. The centre, consisting chiefly of infantry, and commanded by the Duke of Schomberg, was to pass at the fords in front of the Jacobite camp at Oldbridge, where had been collected all James's foot, dragoons, and horse, with the sole exception of Sarsfield's regiment. The left wing, composed exclusively of cavalry, and led by King William in person, was to pass at a ford not far above Drogheda, and flank their foes whilst they were engaged.

James had been prepared for the movement of William's right wing the night before, and he now saw his fatal error in rejecting the advice of his officers to provide against it. He hastily ordered the whole of his left wing, which included Lauzun's French division, with part of his centre, and his six remaining field-pieces, to proceed with all possible expedition to oppose the flanking division; but it was too late to obstruct their passage. The troops under Meinhardt Schomberg marched more rapidly, and the cavalry forced the passage of the river at Rossnaree, which was gallantly defended by Sir Neil O'Neill, who lost seventy of his men and was himself mortally wounded. Portland's infantry and the artillery crossed at Slane, where the bridge had been broken but the river was fordable. Their progress was at first arrested by a morass; but finding, on trial, that though difficult it was not impossible to pass, the infantry marched into it, while the artillery went round by a narrow tract of firm ground at the back of the marshy portion. The Jacobites, astonished, turned and fled, while their opponents, unacquainted with the nature of the ground, advanced stolidly, though slowly and floundering at every step. The cavalry moved more rapidly, and drove before them, with slaughter, all who offered any opposition to their progress.

It was now nearly ten o'clock, and William, having heard that Count Schomberg had succeeded in crossing, ordered the advanced body of his centre to pass the fords. This was composed of the Dutch, the Brandenburghers, the Huguenots, and the Inniskillings. Solme's Blues were the first to move. They advanced with drums beating to the brink of the Boyne, and, marching ten abreast, entered the stream at the highest ford opposite Oldbridge. So shallow was the water here that the drummers only required to raise the drums to their knees. The Londonderry and Enniskillen horse followed, and at their left the Huguenots entered, led by Caillemot, brother of the Marquis de Ruvigny. The English infantry came next, under Sir John Hanmer and the Count Nassau; lower down were the Danes; and at the fifth ford, which was considerably nearer to Drogheda, and at which the water was deeper than at any of the former, William himself crossed with the cavalry of his left wing. Thus was the Boyne, for nearly a mile of its course, filled with thousands of armed men struggling to gain the opposite bank, which bristled with pikes and bayonets. A fortification had been made by French engineers out of the hedges and buildings, and a breastwork had been thrown up close to the water-side. Tyrconnell was there, and under him were Richard Hamilton and the Earl of Antrim.

When the Dutch reached the middle of the river a heavy fire was opened upon them from the breastworks, houses, and hedges, but was ill-directed, and therefore without much effect. As fast as they reached the opposite bank they formed and attacked the Jacobites, who fled from their first defences in the utmost disorder; but as their assailants advanced, fresh troops sprang up from the hedges and ridges behind, and, multiplied to the eye by the manner in which they were disposed, presented a far more formidable appearance than was anticipated. Five battalions bore down upon the Dutch, but they were repulsed. The Jacobite horse were next directed against them, but with no better success, and the Dutch had repulsed two attacks when the Inniskillings and Huguenots arrived to assist them, and drove back with great slaughter a third body of horse.

General Richard Hamilton, who acted throughout the day with marked bravery, and who commanded the Jacobite horse, enraged at the pusillanimity of the foot, distributed brandy among his men, and then led them furiously against the advancing troops, who had now cleared most of the hedges and were ready to form on the unbroken ground. At the same time the French infantry rose suddenly from behind the low hills in the rear, and advanced in good order to support Hamilton's charge. The English centre, confounded at this sudden attack, stood for a moment irresolute. A squadron of Danes, attacked by a part of Hamilton's horse, turned in mid-stream, being followed into the water by their pursuers. The latter then threw themselves upon the Huguenots under Caillemot, who, being unsupported, and having no pikes to withstand the charge, were broken, and their gallant leader ridden down and mortally wounded. Four of his men carried him back across the ford to his tent. As he was borne away he urged on his men, crying to them in French: "To glory, my lads, to glory!"

The Jacobite foot left to defend the ford were, in point of numbers, utterly inadequate, and it is stated that very few of them had muskets, their principal arm being the pike. At the onset they saw themselves unsupported, and they had already suffered severely before the horse came to sustain them, so that under the circumstances they scarcely deserve the epithet of a "mob of cow stealers" which Macaulay bestows upon them. Tyrconnell, who held the chief command in the absence of James, behaved like a gallant soldier; but it would have required more consummate generalship than he possessed to retrieve the fortune of the day. The Jacobite cavalry fought with much valour, the only exceptions being Clare's and Dungan's dragoons; and, the latter having lost their able young commander by a cannon-shot at the commencement of the action, their discouragement was to a certain extent excusable. It was also unfortunate for the Jacobites that Sarsfield's horse accompanied King James as his body-guard, and were thus prevented from taking any part in the action.

Schomberg, who watched the struggle from the northern bank, perceiving the distress of the centre, and learning of the death of Caillemot, now plunged into the river with the impetuosity of a young man, disdaining to don his cuirass, which was pressed upon him by those near him. Without defensive armour he rode through the river to rally the Huguenots, whom the fall of Caillemot had dismayed. "Come on!" he cried in French to the refugees; "there are your persecutors!" pointing with his sword as he spoke to the French squadrons serving under King James. These were his last words. At this moment a troop of Jacobite horse dashed furiously in his direction. He was surrounded and killed, receiving two sabre wounds on the head and a carbine bullet in the neck. About the same time Walker, the gallant Governor of Londonderry, to whom William had just given the See of that city, was also shot dead. The King, who believed that clergymen should confine themselves to such spiritual weapons as the sword of faith and the breastplate of righteousness, when informed that "the Bishop of Derry has been killed by a shot at the ford", contented himself by asking: " What took him there?" The firing had now continued incessantly for about an hour, but it began to slacken amid the general disorder; the tide had begun to run very fast and the passage of the Boyne was becoming more difficult. The battle raged with great fury along the southern shore of the river, the contest being well sustained by both parties; but the Jacobite horse of one wing had to resist unsupported the advance of all the horse and foot of William's left and centre, to which task it was entirely unequal, and they retired, fighting obstinately.

The day was decided by the approach of William, who did not cross the river until late in the action. The advance of the cavalry had been retarded by the unexpected difficulties experienced in crossing, owing to the bottom of the river which was extremely soft, indeed so much so that William's own charger was forced to swim, and was very nearly lost, the King being obliged to dismount and be carried over by his attendants. As soon as he was on firm ground he took his sword in his left hand—his right it will be remembered had been wounded the day before—and led his men to the place where the fight was hottest. They were charged by the Jacobite cavalry with such violence that they were driven back. At this moment William, in the midst of the tumult, rode up to the Inniskillings and cried: "What will you do for me?" He was not immediately recognized, and one dragoon, in the heat of action, mistaking the King's identity, was about to shoot him, when William calmly turned the weapon aside, with the query: "Do you not know your friends?" "It is His Majesty!" cried the Colonel, and the Inniskillings received the announcement with a shout of delight. "Gentlemen," said William, "you shall be my guards to-day. I have heard much of you. Let me see something of you." At length the Jacobite infantry gave way on every side. Hamilton again placed himself at the head of the cavalry and made a desperate attempt to retrieve the future of the day; but though they made a momentary impression they were soon routed, and he was himself severely wounded and taken prisoner.

Meinhardt Schomberg had in the meanwhile been clearing the difficult grounds which had retarded his march, and was now engaged in a pursuit of the troops opposed to him towards Duleek. Long before this an aide-de-camp brought news to James that William's forces had made good their passage at Oldbridge, whereupon the hapless monarch ordered Lauzun to march in a parallel direction with that of Douglas and Count Schomberg towards Duleek, which place he reached before the flying throng of the Jacobite foot. Tyrconnell came up next; and now for the first time the French infantry rendered good service to their side by their admirable discipline, preserving their own order and cooperating with the Jacobite cavalry in covering the retreat. Berwick's horse was the last to cross the narrow pass of Duleek, with the forces of William close in the rear; but beyond the defile the Jacobites rallied. Five of the six field-pieces which James had taken with him in the morning towards Slane were still available; the sixth had stuck in a bog. At the deep defile of Naul the last stand was made. It was now nine o'clock; the fighting had lasted since ten o'clock in the forenoon. The Jacobites drew up in good order and presented a determined front; seeing which it was deemed impolitic for so small a force to attack them, and the order was given for a return to Duleek.